The present invention relates to a photomask used in a lithography process for producing fine patterns of electronic devices such as semiconductor integrated circuits, image pickup devices (CCDs etc.), liquid crystal displays and thin film magnetic heads, and the present invention also relates to an exposure method using this photomask. The present invention is preferably used when light having a wavelength of about 200 nm or less is used as illumination light for exposure.
Conventionally, in a lithography process for forming a fine pattern of an electronic device such as a semiconductor integrated circuit and a liquid crystal display, there is used a method in which a reticle as a photomask on which an original pattern obtained by enlarging a pattern to be formed four to five times is disposed on a projection exposure apparatus, and under predetermined illumination light for exposure (exposure light), the original pattern is reduced in size and projected and transferred onto a wafer (or glass plate or the like) as a substrate to be exposed on which a photoresist is applied.
When such a pattern of the reticle is transferred, a line width of the resist pattern formed on the wafer after development is increased or reduced in accordance with integrated exposure amount of the exposure light with respect to the wafer. Thereupon, in order to obtain a designed line width over the entire surface of the resist pattern, the illumination distribution of the exposure light to the pattern of the reticle is maintained uniform extremely precisely so that an error of the distribution is in a range of, for example, +1% within the illumination region.
In the projection exposure apparatus, in order to enhance resolution to meet finer semiconductor integrated circuits, wavelength (exposure wavelength) of the exposure light tends to be shorter. At the present, 248 nm of a KrF excimer laser becomes mainstream as the exposure light wavelength, but 193 nm of an ArF excimer laser having shorter wavelength will soon be in practical use. Further, research has been conducted to develop a projection exposure apparatus using an F2 laser having shorter wavelength (wavelength is 157 nm).
If the exposure light wavelength is in vacuum ultraviolet region (VUV region) of wavelength of about 200 nm or less, e.g., 157 nm by the F2 laser, kinds of preferable materials as substrate material of the reticle which allow exposure light of such short wavelength to pass therethrough are limited. For example, fluorite (calcium fluoride) has excellent transmissivity at that wavelength, but since the linear expansion coefficient is as great as about 20xc3x9710xe2x88x926/K, the fluorite is not always preferable as the substrate material of reticle. With such a great linear expansion coefficient, the substrate of the reticle is expanded by the illumination heat of the exposure light generated when the exposure and transfer, and the positional precision of a pattern to be transferred is deteriorated. Therefore, in order to use the fluorite, it is necessary to enhance the cooling function of the reticle for example.
As explained above, in the projection exposure apparatus, the exposure light wavelength tends to be shorter, but if the exposure light wavelength becomes about 157 nm, there is conventionally almost no material for the substrate of the reticle having high transmissivity and relatively small linear expansion coefficient.
In this regard, an attempt to use quartz and a synthetic quartz (quartz glass) doped with fluorine as the substrate material which is substantially transparent with respect to light of wavelength of about 157 nm has been made.
However, also with respect to the quartz and the quartz glass doped with fluorine, the wavelength of about 157 nm is close to the absorbing end (wavelength from which material-inherent abrupt absorption starts) of light for each material, and there is an adverse possibility that the transmissivity distribution of the material is largely varied at the wavelength near 157 nm due to slight non-uniformity in composition of the material, stress deformation generated in the material or the like. Due to these factors, if the transmissivity distribution inside the reticle substrate becomes uneven (non-uniformity), the uniformity in line width of the resist pattern to be transferred onto the wafer is deteriorated as in the case in which the illumination distribution of the exposure light for illuminating the reticle becomes non-uniform. The deterioration in uniformity in line width of the resist pattern brings about non-uniformity in circuit line width in an electronic device to be produced, which largely deteriorates operating speed of the electronic device for example.
In view of the above circumstances, it is a first object of the present invention to provide a substrate for a photomask having highly uniform transmissivity distribution and to provide the photomask.
It is a second object of the invention to provide a substrate for a photomask having a. relatively high transmissivity with respect to light of short wavelength of about 200 nm or less, for example, and having a uniform transmissivity distribution, and to provide the photomask.
It is a third object of the invention to provide an exposure method and a producing method of a device capable of producing an advanced device using the photomask.
A substrate for a photomask according to the present invention is a substrate for a photomask on which an original pattern is formed, and the substrate is provided with a transmisslvity compensating member which compensates for non-uniformity of a transmissivity distribution inside the substrate.
According to the present invention, if the photomask is irradiated with exposure illumination light (exposure light) of a predetermined wavelength, a reduction amount of the transmissivity by the transmissivity compensating member is reduced in a portion of the substrate where the transmissivity of the substrate itself is lower than the average value under that wavelength, and the reduction amount of the transmissivity by the transmissivity compensating member is increased in a portion of the substrate where the transmissivity of the substrate itself is higher than the average value. With this arrangement, the non-uniformity of the transmissivity distribution of the substrate is compensated, and it is possible to secure sufficiently high uniformity of the transmissivity distribution to be used as a substrate for the photomask.
In this case, it is preferable to. form the substrate of quartz, quartz glass (e.g., synthetic quartz glass having hydroxyl (OH group) of concentration of 1000 ppm or more), quartz glass doped with a predetermined impurity (e.g., fluorine (F2) or the like), sapphire (Al2O3) or magnesium fluoride (MgF2). These materials have relatively high transmissivity even in vacuum ultraviolet region of wavelength of about 200 nm or less, and since the quartz to the sapphire have linear expansion coefficient smaller than that of fluorite, the quartz to the sapphire are suitable as the substrate of the photomask which is irradiated with exposure light of such short wavelength. The magnesium fluoride can allow light of shorter wavelength to pass through as compared with the fluorite.
One example of the transmissivity compensating member is a thin film provided on one surface of the substrate, and a film thickness distribution of this thin film is set in accordance with the transmissivity distribution of the substrate. In this case, the transmissivity distribution of the substrate can be compensated only by controlling the film thickness distribution.
The transmissivity compensating member may be formed by reforming, in at least one surface of the substrate, a vicinity of the one surface of the substrate, or the transmissivity distribution may be formed by providing another substrate other than the substrate with a transmissivity distribution which substantially compensates for the non-uniformity of the transmissivity distribution of the substrate.
Next, a first photomask according to the present invention is a photomask which includes a substrate having a thin film as the transmissivity compensating member, and an original pattern is formed on a surface of the substrate opposed to a surface on which the thin film is formed. With this arrangement, the original pattern can be formed without being affected by the transmissivity compensating member.
A second photomask according to the present invention is a photomask which has a substrate provided with the transmissivity compensating member according to the invention, and the original pattern is formed on one surface of the substrate. As this photomask, a mask having a substrate doped with a predetermined impurity with such a distribution as to compensate for transmissivity distribution of the substrate itself is included.
It is preferable that a line width of at least one portion of the original pattern is different from the designed value in accordance with the transmissivity distribution of the substrate provided with the transmissivity compensating member.
In the present invention, the designed value is a value obtained by multiplying a size of a pattern to be formed on a photosensitive object by a reciprocal of projection magnification of a projection optical system when the original pattern of the photomask is transferred onto the photosensitive object through the projection optical system. When the original pattern (including both dense pattern and isolated pattern) is transferred onto the photosensitive substrate, if the line width of the pattern to be actually formed on the photosensitive substrate becomes thin or short with respect to a size value of a pattern to be formed on the photosensitive substrate, the size value of the original pattern is increased or reduced to correct to compensate the variation amount in some cases, and a value after the correction is also included in the designed value of the invention.
Next, in a third photomask according to the present invention, an original pattern is formed on the substrate and in order to compensate for the non-uniformity of the transmissivity distribution inside the substrate, a line width of each pattern in the original pattern is changed in accordance with the transmissivity distribution of the substrate. With this photomask, in a portion thereof where the transmissivity of the substrate itself is lower than the average value for example, the line width of a light-shield pattern in the original pattern is made thinner than the designed value, and in a portion where the transmissivity of the substrate itself is higher than the average value, the line width of the light-shield pattern is made thicker than the designed value. With this arrangement, the transmissivity distribution of the photomask is uniformized.
Next, in a first exposure method according to the present invention which illuminates a photomask to expose a photosensitive object with light passing through an original pattern of the photomask, the substrate is provided with a transmissivity compensating member in order to compensate for non-uniformity of a transmissivity distribution inside a substrate of the photomask. That is, the first or second photomask of the present invention is used.
In a second exposure method according to the present invention which illuminates a photomask to expose a photosensitive object with light passing through an original pattern of the photomask, in order to compensate for nonuniformity of a transmissivity distribution of the photomask, an illumination distribution of light on the photosensitive object is adjusted in accordance with the transmissivity distribution.
With the second exposure method of the present invention, the illumination distribution of the light on the photosensitive object is adjusted in accordance with the transmissivity distribution of the photomask, and the uniformity of the illumination distribution of the light on the photosensitive object is enhanced, thereby improving the uniformity of the line width of the pattern formed on the photosensitive material.
In a third exposure method according to the present invention which illuminates a photomask to expose a photosensitive object with light passing through an original pattern of the photomask, in order to compensate nonuniformity of a transmissivity distribution inside a substrate of the photomask, a line width of each pattern in the original pattern is changed in accordance with the transmissivity distribution of the substrate. With this arrangement, the third photomask of the invention is used.
Next, a first device producing method in accordance with the present invention comprises transferring a device pattern onto a substrate for a device using the exposure method of the present invention. A second, third or fourth device producing method according to the present invention is a device producing method for producing a predetermined device using the first, second or third photomask of the present invention, and comprises exposing an original pattern on the substrate onto a device substrate by illuminating the photomask with illumination light passing through the substrate. The uniformity of the transmissivity distribution of the photomask according to the present invention is extremely high and therefore, the uniformity of the line width of the circuit pattern of the device formed on the substrate is enhanced, and an advanced device can be produced.